Traditional digital images from low resolution sources, such as, cellular telephones, personal digital assistants and web sites typically have both compression artifacts and image noise. The compression artifacts often produce blocks and ringing, and the image noise often results in correlated luminance and chrominance errors. Images that contain both compression artifacts together with image noise are much harder to enhance than images containing only a single type of degradation. This is because estimating image noise levels is difficult for low-resolution images, and erroneous noise estimates in the luminance channel often result in insufficient luminance noise reduction or overly blurred images, and artifact reduction alone does not sufficiently reduce the strong chrominance noise (which is not necessarily due to compression) that is typically observed with these images.
In comparing the lower resolution images obtained from the above-described low resolution sources with higher resolution images, the lower resolution images typically have higher noise due to low quality CMOS image sensors. In addition, the lower resolution capture devices typically have lower cost, and hence lower quality optics, and often have a relatively high compression ratio. As such, the lower resolution images often have a relatively large number of visually objectionable artifacts, especially for images obtained in low light situations, which are typically more clearly visible when the images are enlarged.
Conventional techniques for improving the lower resolution images have included aggressive linear filtering. However, conventional aggressive linear filtering techniques often resulted in the introduction of visible saturation variations or excessive de-saturation of the image. Other challenges to conventional image enhancement techniques for low resolution images include the fact that many artifact reduction methods do not work reasonably well for chrominance channels because chrominance noise and artifact are not due solely to compression artifacts, but due to other image noise sources, and they are found to be extremely high for many of the low-resolution images. In addition, it is often relatively difficult to estimate the amount of noise and hence the denoising parameters for relatively small images. Moreover, estimating chrominance channel compression parameters is typically much more difficult than the luminance channel because of unknown subsampling factors and higher compression settings for chrominance.